Last weekend I went to Geneva, to visit Ciprian, a friend who is doing an
internship at CERN. He also happens to be a frequent reader of my blog :D.

Before joining the darkcomputer science side, I was a wannabe
physicist. I went to lots of physics com­pe­ti­tions (and had some good results)
and I even applied to the physics department at UBB. But in 12th grade I
discovered that pro­gram­ming is also quite fun, and it has a greater payoff, so I
decided to go study that. I don't regret this decision, but I still fondly
remember the things that I learned from my awesome physics teachers and when I
have a chance to solve a physics problem, it still gives me thrills.

So I was super excited to go visit CERN, home of the largest physics experiment
in the world, the Large Hadron Collider. And it didn't dissapoint. The sheer
scale of the things they are doing there is over­whelm­ing, from the coldest place
in the Solar System, to the hottest place in the universe, to the highest degree
of vacuum in the Milky Way, it's all here. It's a remarkable feat of extremely
reliable en­gi­neer­ing (don't forget, my job title is Site Re­li­a­bil­i­ty Engineer,
so I've recently gained an ap­pre­ci­a­tion for large systems that work reliably :D).

The tour started at 2PM. I was in a group with students from several
high­schools, and some other in­di­vid­u­als. The guide took us to an auditorium
where he gave us a short pre­sen­ta­tion about the history of CERN and some of its
major ac­com­plish­ments. Then we went over to the French side to SM-18, one of the
testing facilities for the su­per­con­duct­ing magnets. Here the fun facts began.

In order to be able to steer the protons flying around at 99.999991% of the
speed of light, they have to use very strong magnets, generating a field of 8.4
Teslas (which is 100000 times more powerful than Earth's magnetic field). But
fer­ro­mag­nets are not strong enough, so they have to use electro-magnets. But
generating 8T of magnetic fields requires 11000 A (yes, eleven thousand) of
current flowing through the wires. That is an insane amount of current and would
generate an enormous energy loss because of electric resistance. So they have to
use su­per­con­duct­ing materials. But all of the su­per­con­duct­ing materials known
today operate only when cooled down way below 0 Celsius, so the magnets need to
be cooled down. They use niobium-titanium for the wire, which needs to be cooled
down to 4 Kelvin to become su­per­con­duct­ing. This material was chosen because of
other physical properties such as elasticity. In order to keep everything so
cold, they use liquid hydrogen. But nature sometimes likes to play games and
oc­ca­sion­al­ly, on very small portions, the su­per­con­duct­ing wires lose their
su­per­con­duc­tiv­i­ty and they start heating up right away, which would turn the
helium into a gas, increase the pressure and make an explosion. So, they choose
to cool the helium down to 1.9 Kelvin (colder then outer space), so that it
becomes a superfluid. In this state, a liquid has zero viscosity, it "climbs"
walls, so it fills all the available nooks and crannies, and it also becomes an
excellent heat conductor (actually it does convection). This means that even if
a small piece of wire starts heating up, the sur­round­ing helium will transport
away all the excess heat and keep things safe. Because there are two beams,
which travel in opposite directions, they need two magnetic fields, oriented
dif­fer­ent­ly. Because 2x8 T magnetic fields would interact very strongly, the two
beams have to be encased in a thick steel shielding. This also has the advantage
of being Fer­ri­mag­net­ic, so that there is no magnetic field outside of the steel
casing.1

After this we went to the ATLAS experiment and we saw the control rooms, where
the operators are over­look­ing everything. When the machine is running, there is
always someone on-call. But many decisions have to be made much faster than a
human can, so they have lots of automated systems keeping things under control.

Then we had a short 3D video (made by post­pro­cess­ing 2D footage :( ) presenting
the operation of the LHC. The protons are extracted in Linac 2, a linear
ac­cel­er­a­tor. At one end, a bottle of hydrogen gas feeds atoms into it, while an
electric field strips off the electrons. When they reach the other end of Linac
2, the protons have an energy of 50 MeV and have gained 5% in mass. From here
they go into the Proton Synchroton Booster, where they go up to 1.4 GeV in
energy. The particles are then injected into the Proton Synchroton (without
booster), which is the first ac­cel­er­a­tor built at CERN, having first worked in
1959. It has a cir­cum­fer­ence of 628 metres, has been improved and upgraded many
times over the years, and now it can accelerate particles up to 25 GeV. The next
in this long chain of ac­cel­er­a­tors is the Super Proton Synchroton (start to see
a pattern?), which has a cir­cum­fer­ence of 7 km, and it brings up protons to 450
GeV. And from here we finally arrive to the Large Hadron Collider (sorry, no
pattern here), a 27 km long ring, where particles are ac­cel­er­at­ed to 4 TeV into
two directions and then they collide, having a total collision energy of 8 TeV.
Protons travel together in 30 cm long bunches, that contain 10^11 particles, so
the total energy of a bunch is about 133 kilo Joules, which is about a tenth of
the energy of a car going on the highway at 130 km/h, but con­cen­trat­ed in a much
smaller space, of 0.2 mm. The total energy of a beam, which has 2808 bunches in
it, is equivalent to a 400 tonne TGV travelling at 150 km/h and is enough to
melt 500 kg of copper. When the protons are prepared for collisions, the beam
is actually focused to 16 microns.

When the two beams travelling in opposite directions collide, many new
particles are created. I really like the analogy given by the guide: it's like
taking a hammer to a watch. If you hit it lightly, the glass will crack and you
will get small pieces of glass. If you hit it harder, you will get more of the
insides. If you hit it really hard, you will decompose the watch to all of it's
mechanical pieces. By colliding protons, we sometimes get some really rare
particles, some of which are not seen anymore in the Universe and they are
speculated to have been around at the Big Bang. This will hopefully lead us to
understand better how gravity and quantum mechanics fit together, why is there
more matter then antimatter (even though whenever we create antimatter, we
create the same amount of matter too), and, as most scientists hope, it will
lead to something completely unexpected, which will turn our theories on their
head and open up completely new fields.

The collisions are recorded with what is basically a fancy-shmancy camera, one
with 100MPs and that can take 40 million images per second. It has three parts,
which all have different resolution and can track different particles (here they
were starting to lose me in the pre­sen­ta­tion). The really cool thing is that
when the particles start colliding, they generate 1 Petabyte of in­for­ma­tion per
second. This is way too much to be stored, so they filter out all the know
events, or any that don't seem "in­ter­est­ing" with FPGAs and ASICs that are
located close to the experiment sites, so that only 1 out of 10000 events is
sent to the server farm, where some more advanced algorithms filter out so that
only 1% is actually stored and then analyzed.

CERN has their own data center, which is also a museum of sorts for all kinds of
old device, including one of the NEXTCUBE computers which served as the first
web server :D

Cipri gave me a private tour of the SC as well, where there is a super awesome
projection mapping, that overlays on top of a section of the old
Syn­chro­cy­clotron how it was assembled, how it worked and what kind of results it
obtained. Un­for­tu­nate­ly, because it was closed for 20 years, the purpose of many
of the tools and hardware that is there is no longer known :))

After this, Cipri and I went to a barbecue at some of his CERN friends. This was
really nice because I got to hear a lot of stories that you wouldn't normally
hear from a guide :)) (and I also found out that "mici", a tra­di­tion­al Romanian
grilled meat, is called "c­se­vap­c­sic­sa" in other languages). They told me about
the UFOs (Uniden­ti­fied Falling Objects) that plague CERN. It is not yet very
well known what are these things, but they often cause problems by in­ter­fer­ing
with the beam and causing it to become unstable, so it has to be dumped. Dumping
the beam is done by letting it go into a large slab of concrete and graphite,
and there are rumours of the sound it makes as it slams into it. Another tale is
how when the engineers from CERN went to the worlds largest magnet man­u­fac­tur­er
and asked for a 35 tonne, 15 meter long magnet, they were basically laughed out
of the building. So they had to design it themselves, make a prototype, and then
go back saying "If we give you the prototype, can you make us 1232 more magnets
like this?". Another story was when they were looking for a way to reduce the
heat losses of the vacuum chambers that are in the tunnel, they developed a
reflective surface with which to coat the insides, so that there is as little
heat exchange as possible. But went they sent it for painting, there was a
mis­com­mu­ni­ca­tion and they got it back painted on the outside. It was a very nice
paint job, very smooth, very uniform, just on the wrong side.

I'm very happy that I got this chance to visit CERN, as it was a dream of mine
for a long time. I'm even more amazed by what the scientists and engineers do
there, at what scale they work, and to be honest, maybe one day I would still
like to work there.